Diffusion-weighted (DW) MRI enables qualitative and quantitative assessment of tissue diffusivity without the use of contrast agent. While such method has gained great success in the brain, DW MRI remains challenging in the body (e.g., for liver imaging) particularly at 3T and higher field strengths. Diffusion-sensitizing gradients can lead to substantial signal loss in the targeted organ due to body motions (e.g., breathing, heart beating) beyond diffusion. Meanwhile, the conventional single-shot EPI diffusion sequence is challenged by limited spatial resolution, increased image distortion, and ghosting artifacts at high field strength. Previous work demonstrated that a first-order moment (M1) nullified gradient module significantly increases the tolerance of diffusion preparation to cardiac and respiratory motions. Furthermore, stack-of-stars sampling scheme has been shown to be a motion-robust method and was successfully utilized for free-breathing body imaging.
MRI has been increasingly used for the diagnosis of patients with body diseases due to distinct advantages of MRI when compared to competing imaging modalities: 1) MRI provides excellent soft tissue contrast; 2) MRI procedures are free from ionizing radiations. For oncological imaging, diffusion-weighted (DW) MRI enables qualitative and quantitative assessment of tissue diffusivity without the use of exogenous contrast media. Although readout-segmented EPI sequence alleviates some of these challenges, its performance can still be compromised by motion and mis-registration between multiple readout segments. For diffusion imaging in the body, its efficiency is further compromised by segmenting readout and acquiring additional navigator for respiratory motion control.
Recent work has demonstrated that diffusion-prepared sequence can be used for imaging prostate cancer patients undergoing active surveillance protocol for low-risk prostate cancer. For moving organs such as the heart, a first-order moment (M1) nullified gradient module significantly increases the tolerance of diffusion preparation to motions. In combination with navigator-gating and electrocardiogram triggering, the feasibility of diffusion-weighted imaging has been demonstrated in the heart using such a motion compensated diffusion preparation scheme. From a data acquisition perspective, a stack-of-stars (SOS) sampling scheme was also shown to be a motion robust method and was successfully utilized for free-breathing, contrast-enhanced multiphase imaging of the liver. Such method was also used for improving the delineation of carotid vessel wall that was challenged by pulsation of carotid arteries and swallowing of a subject during data acquisition.